Flexible circuits are circuits that are formed on flexible dielectric substrates such as polymeric materials. The circuits may have one or more conductive layers as well as circuitry (also called “traces”) on one or both of the major surfaces of the substrate. The circuits often include additional functional layers, e.g., insulative layers, adhesive layers, encapsulating layers, stiffening layers and the like. Flexible circuits are typically useful for electronic packages where flexibility, weight control and the like are important. In many high volume situations, flexible circuits also provide cost advantages associated with efficiency of the manufacturing processes employed.
Various types of flexible circuits are known in the industry. Generally speaking, the key differences in the various circuits stem from a number of design requirements for the devices that the circuit is connecting together, along with the requirements and limitations of the processing methods used to make the circuit. Typically, the flexible circuit is connecting a semiconductor device of some sort (integrated circuit, microprocessors, or the like) to another flex circuit, a rigid circuit board or a component of a device through connection means such as bond pads, solder balls, etc. The design factors associated with items the circuit is connecting include, but are not limited to, the number of input and output (I/O) leads from a semiconductor device that needs to be connected; the means and process for interconnection of the flexible circuits to another circuit or to a device; the required size and weight of the finished product; the environmental conditions under which the circuit will be assembled and used; and the data transmission rates to which the circuit will be subjected.
Flexible circuits have become widely used in telecommunications equipment and consumer and industrial electronic appliances. As the packaging of those appliances becomes simpler, more compact, more reliable, and more highly functional, requirements imposed on flexible printed circuit boards become extremely stringent. The boards are required to have high thermal resistance, good weatherability, electric insulation properties, bonding strength, and flexibility, and to meet severe conditions, including limits on tribocharging of the electronic packages.
Polymeric films, such as those used in flexible circuits, are typically electrically insulating and exhibit a propensity towards the generation of electrical charges (referred to hereinafter as tribocharging). Tribocharging is undesirable for a number of reasons. Contaminants such as dust particles are attracted by charged surfaces. Tribocharges are also known to damage various types of electronic devices, such as semiconductor devices, due to electrical currents associated with the discharge of accumulated tribocharge. Therefore, what is needed is a flexible circuit construction that reduces tribocharging on the surface of the circuit such that the potential for damage to electrical components connected to the circuit is reduced.
Protective films coated or placed onto the surface are one known construction. Known surface protective films of flexible wiring circuits are used to protect against cracking and contamination as well as tribocharging, and include, for example, those prepared by cutting polyimide films using a die made corresponding to the pattern, which are then adhered onto a substrate with an adhesive; and those prepared by applying a radiation-curable or thermosetting overcoat composition. Resin compositions used as protective films include epoxy resins, acrylic resins, and the like. However, such resins only provide minimal protection against tribocharging. Further, most such resins must be punched or drilled in order to provide access through the protective film to the circuitry.
It would be desirable to reduce the tribocharging of the electrical components to a range of about 15 Volts (V), preferably less than 10V.
It has now been discovered that a coating comprising a polypyrrole not only reduces tribocharging significantly but that bonding of electronic components to traces on the flexible circuit can be made through the coating, simplifying production and connection of the flexible circuit to such electronic components.